1. Field of the Invention
The present invention relates to a heat developing apparatus for heating a heat developing photosensitive material or a photosensitive and thermosensitive recording material to perform heat development.
2. Description of the Related Art
As an image recording apparatus for recording an image for medical use in, for example, a digital radiography system, a CT or an MR, a wet system has been used with which an image is photographed or recorded on a silver-salt photosensitive material and after which a wet process is performed to obtain a reproduced image.
On the other hand, a recording apparatus in the form of a dry system with which no wet process is performed has attracted attention in recent years. A recording apparatus of the foregoing type uses a photosensitive and thermosensitive recording material (a photosensitive and thermosensitive recording material) or a heat developing photosensitive material (hereinafter called a “heat developing recording material”). The dry recording apparatus incorporates an exposing portion in which the heat developing recording material is irradiated with a laser beam (that is, scanned) so that a latent image is formed. Then, the heat developing portion brings the heat developing recording material into contact with a heating means so that heat development is performed. Then, the heat developing recording material is cooled, and then the heat developing recording material having an image formed thereon is discharged to the outside of the apparatus.
The heat development performed by the heat developing portion is performed such that the heat developing recording material, the temperature of which is substantially the room temperature, is heated to a level not lower than the heat development start temperature. Then, the temperature is maintained for a predetermined period of time. Hitherto, a heat developing apparatus incorporates a heating means, such as hot air, infrared rays, a heating drum, a heating plate or a heating roller, for heating and developing the heat developing recording material has been known (Japanese Patent Application No. 9-229643).
However, heat conduction in the heat developing portion cannot always uniformly be performed. Therefore, the heat hysteresis differs among the portions of the heat developing recording material. As a result, there is apprehension that irregularity of development occurs. The reason why the heat conduction cannot uniformly be performed has been detected as follows.
The cause of the problem experienced with the conventional apparatus will now be described with reference to FIG. 2.
Referring to FIG. 2, FIG. 2(a) is a diagram showing change in the state of the heat developing recording material placed on a heating plate which is the heating means, the change occurring as time elapses.
FIG. 2(b) is a graph showing change in the temperatures at points A and B of the heat developing recording material occurring as time elapses.
FIG. 2(c) is a diagram showing the sizes (the densities) of recorded dots developed on the points A and B.
Referring to FIG. 2, the heat developing recording material, the temperature of which is the room temperature, is placed on the heater so as to be heated. In a state at time t1 shown in FIG. 2(a), the irregularity of the thickness of the base of the heat developing recording material, that of the density of the material of the base, a state of asperities of the heat developing recording material and irregularity of the heat distribution of the heater causes portions which are intensely heated and portions which are heated weakly to be realized.
As a result, the portions of the heat developing recording material heated intensely (the point A of the heat developing recording material) are quickly heated to the glass transition temperature TG (see FIG. 2(b)). On the other hand, portions of the heat developing recording material heated weakly (the point B of the heat developing recording material) are slowly heated to the glass transition temperature TG.
The portions (the point A) of the heat developing recording material quickly heated to the glass transition temperature lose rigidity quickly. Therefore, the foregoing portions are brought into furthermore hermetic contact with the heater so that greater heat is exerted to the foregoing portions. Therefore, the portions are quickly heated to the development start temperature TH (time t1 shown in FIG. 2(b)). On the other hand, the portions of the heat developing recording material which are heated weakly (the point B) are slowly heated to the glass transition temperature TG. Therefore, the foregoing portions are slowly heated to the development start temperature at time t2 shown in FIGS. 2(a) and (b).
Assuming that the time at which the development of the points A and B is stopped is t4 (see FIG. 2(b)), development time tA at which the point A is developed is expressed by equation (1) and development time tB at which the point B is developed is expressed by equation (2).
 tA=t4−t1  (1)tB=t4−t2  (2)Since tA and tB satisfy equation (3)tA>tB  (3)
Therefore, the sizes of the recorded dot of the points A and B are as shown in FIG. 2(c). Thus, the point A is formed into a larger dot and has a high density as compared with the point B. Therefore, irregular development takes place over the whole heat developing recording material.
To achieve the foregoing problem, an object of the present invention is to provide a heat developing method with which occurrence of irregularity of heat development can be prevented over the surface of the heat developing recording material and an apparatus therefor.